Polymer dispersed liquid crystal electro-optical device and method for manufacturing the same

ABSTRACT

A polymer dispersed liquid crystal electro-optical device comprising a liquid crystal polymer complex layer having a liquid crystal and a polymer, wherein said liquid crystal and said polymer are aligned in the same direction when no electric field is applied, and an electrode structure formed on each side of said liquid crystal polymer complex layer for applying an electric field to said liquid crystal polymer complex layer to align said liquid crystal along the electric field so as to render said liquid crystal polymer complex layer in a light-scattering state. The liquid crystal polymer complex layer is formed by dissolving a liquid crystal and a polymer precursor to form a solution; adding a compound in which at least one hydrogen atom in a benzene ring is substituted by a hydroxy group or a compound having a benzene ring with a hydroxy group as a basic skeleton to said solution; polymerizing said polymer precursor to from a polymer; and phase separating said liquid crystal and said polymer to form a liquid crystal polymer complex layer. The compound improves the initial refractive index, and the refractive index and contrast after energize-aging.

FIELD OF THE INVENTION

The present invention relates to a polymer dispersed liquid crystalelectro-optical device, in which a liquid crystal and a polymer arecompatibly dispersed, and a method for manufacturing the same.

DESCRIPTION OF RELATED ART

Conventionally, various types of display devices using a liquid crystaldisplay have been popularized, and a majority of the devices employ aliquid crystal display according to a so-called TN (Twisted Nematic)type or a STN (Super Twisted Nematic) type. However, since these liquidcrystal displays must use two polarizing plates, as is widely known,efficiency of light availability decreases, thereby disadvantageouslycausing a dark display.

Therefore, polymer dispersed liquid crystal display devices have beenrecently developed that do not need polarizing plates by utilizing adifference between a liquid crystal and a polymer. These polymerdispersed liquid crystal electro-optical devices are prepared such thata liquid crystal and a polymer are compatibly dispersed andphase-separated to form a liquid crystal polymer complex layer. Thesedevices are in a transparent state in which light can be transmittedwhen the refractive indexes of the liquid crystal and the polymer havethe same value due to application or removal of an electric field to orfrom the above-mentioned complex layer. These devices are in a cloudystate (translucent state) in which light is scattered if the refractiveindexes are different from each other.

In these devices, a liquid crystal is dispersed in a polymer and theliquid crystal and the polymer are phase-separated to form a liquidcrystal polymer complex layer under a condition in which liquid crystalmolecules having positive dielectric anisotropy are aligned at randombetween two transparent substrates, as is described in U.S. Pat. No.3,600,060 in detail. When no electric field is applied (without appliedelectric field), the refractive indexes of the randomly aligned liquidcrystal and the polymer also aligned at random differ from each other,giving a light-scattering state in the liquid crystal polymer complexlayer. Moreover, when an electric field is applied, the liquid crystalis aligned in the direction of the electric field by the electric field,and thus, the difference between the liquid crystal and the polymerdisappear so that the liquid crystal polymer complex layer is renderedin a transparent state.

According to such polymer dispersed liquid crystal display devices,micro-particles of a liquid crystal are dispersed in a polymer, andfurther, the polymer is aligned at random. Thus, sufficient lighttransmittance cannot be obtained even when the liquid crystal and thepolymer have the same refractive index, resulting in an incompletetransparent state. In addition, since the liquid crystal has an unevenparticle diameter, the display quality is not uniform, leading toreduced reliability. Moreover, the response of each liquid crystalmolecule to an electric field is nonuniform because the liquid crystalis also aligned at random when no electric field is applied. Thus, aproblem occurs in that the threshold characteristics of transmittance ofthe device as a whole is not rapid.

Therefore, as is described in Japanese Patent Laid-Open No. 5-119302 andthe like, a polymer dispersed liquid crystal display device has beenproposed according to a system in which a sufficient light-transmittingstate is achieved when no electric field is applied because a liquidcrystal and a polymer are aligned in the same direction so that theyhave almost the same refractive index. A light-scattering state isachieved when an electric field is applied because only the liquidcrystal is aligned in the direction along the electric field so that therefractive indexes of the liquid crystal and the polymer differ fromeach other. According to the above system, a transparent state superiorto conventional ones can be obtained. In particular, a superiorlight-scattering state can also be obtained when applying an electricfield by forming a twisted structure in which a liquid crystal and apolymer are twisted. Further, a liquid crystal display device havinghigh quality and improved threshold characteristics can be realized.However, even in the above-mentioned improved polymer dispersed liquidcrystal display device, a problem occurs in that the scatteringperformance is influenced by the materials used. In addition, concerningreliability, there is a problem in that a phenomenon (hereinafterreferred to as "afterimage") occurs when an electric field is removedafter being applied for a long time period, in which phenomenon thealignment direction of the liquid crystal does not relax to its originaldirection and the electric field-applied state, i. e., thelight-scattering state, partially remains. In this case, afterimage isoccasionally improved to some extent by mixing a bifunctional monomerhaving two functional groups in monomers which are used as precursors ofa polymer to be compatibly dissolved in the liquid crystal. However,stable effects cannot be obtained.

In addition, different from the above-mentioned afterimage, otherproblems occur such that the transparent state deteriorates with thelapse of time and contrast decreases when the liquid crystal displaydevice is exposed to high temperatures or electric fields are repeatedlyapplied to and removed from the device.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a polymer dispersedliquid crystal electro-optical device and a method of manufacturing thesame, which polymer dispersed liquid crystal electro-optical deviceachieves improved scattering performance and prevents afterimage anddeterioration in the transparent state with the lapse of time.

A polymer dispersed liquid crystal electro-optical device comprises aliquid crystal polymer complex layer formed by compatibly dispersing aliquid crystal and a polymer and phase-separating the liquid crystal andthe polymer, in which the liquid crystal and the polymer are aligned inthe same direction when no electric field is applied; and an electrodestructure formed on each side of the liquid crystal polymer complexlayer for applying an electric field to the liquid crystal polymercomplex layer to align the liquid crystal along the electric field so asto render the liquid crystal polymer complex layer in a light-scatteringstate. The liquid crystal polymer complex layer is formed bypolymerizing a polymer precursor under such conditions that a compoundin which at least one hydrogen atom in a benzene ring is substituted bya hydroxy group or a compound (hereinafter occasionally referred to as"additive") having a benzene ring with a hydroxy group as a basicskeleton is added to a solution compatibly dissolving the liquid crystaland the polymer precursor.

According to the polymer dispersed liquid crystal electro-opticaldevice, the addition of the above-mentioned additive can significantlyimprove scattering performance during energizing as compared with thatof devices not containing the additive, and also suppressesdeterioration of a light-scattering state and a transparent state afterenergizing for a long time period, thereby improving display contrast.

It is preferred that the above mentioned compound in which at least onehydrogen atom in a benzene ring is substituted by a hydroxy group or thecompound having a benzene ring with a hydroxy group as a basic skeletonis ditert-butylhydroquinone. The added amount of this compound is easilyadjusted, since a great improvement is achieved in the displaycharacteristics by its addition, and its solubility in theabove-mentioned solution is relatively high.

Furthermore, the added amount of the above-mentionedditert-butylhydroquinone with respect to the above-mentioned solution ispreferably 0.0005 to 1% by weight, and more preferably, 0.001 to 0.1% byweight. When the added amount is within this range, the scatteringperformance can be particularly improved. Also, the refractive index andcontrast after energizing for a long time period can be raised higherthan that without addition of ditert-butylhydroquinone, in addition tosuppressing reduction in polymerizing speed or an increase in residualmonomer.

Hydroquinone can also be used for the above-mentioned additives. Theadded amount of hydroquinone with respect to the above-mentionedsolution is preferably 0.0005 to 1% by weight, and more preferably,0.005 to 0.1% by weight. By using hydroquinone within theabove-mentioned range, heat polymerization of a polymer precursor can besuppressed, and the electro-optical characteristics and reliability ofthe electro-optical device produced is thereby improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an enlarged fragmentary sectional diagram showing a structureof an embodiment of a polymer dispersed liquid crystal electro-opticaldevice according to the present invention;

FIG. 2 is an enlarged sectional diagram showing a structure under anenergizing condition in the embodiment shown in FIG. 1;

FIG. 3 shows the relationship between the refractive index under anenergizing condition and the added amount of BHQ in a polymer dispersedliquid crystal electro-optical device of Example 1 incorporated in thepresent invention after treating with heat-aging and energize-aging;

FIG. 4 shows the relationship between contrast and the added amount ofBHQ in Example 1 after treating with heat-aging and energize-aging;

FIG. 5 shows the relationship between the amount of residual monomer andthe polymerizing time for each added amount of BHQ in Example 1;

FIG. 6 shows the relationship between the added amount of BHQ and thedriving voltage in a polymer dispersed liquid crystal electro-opticaldevice of Example 2 according to the present invention;

FIG. 7 shows the relationship between the added amount of BHQ and the ONrefractive index in Example 2;

FIG. 8 shows the relationship between the added HQ amount and thedriving voltage in a polymer dispersed liquid crystal electro-opticaldevice of Example 3 according to the present invention; and

FIG. 9 shows the relationship between the added HQ amount and therefractive index in Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, a polymer dispersed liquid crystal electro-optical device and amethod for manufacturing the same according to the present inventionwill be explained in more detail with reference to the accompanyingdrawings. The following embodiments relate to cases in which a polymerdispersed liquid crystal electro-optical device according to the presentinvention is applied to a reflective type liquid crystal display. Thepresent invention is applicable to either transmissive types orreflective types and also can be applied to not only liquid crystaldisplays but also to other devices such as electro-optical shutters.

FIG. 1 is a sectional diagram showing a liquid crystal display of thisembodiment. A transparent electrode 3 composed of ITO (Indium TinOxide), etc. is formed on the surface of a first transparent substrate 1by depositing, sputtering, or the like. Meanwhile, on the surface of asecond transparent substrate 2, a metallic electrode 4 is similarlyformed which is composed of metals such as aluminum, chromium, etc. andwhich also serves as a reflective layer. A first alignment layer 5 and asecond alignment layer 6, both of which are made of polyimide, polyvinylalcohol, etc., are formed on the surface of the first transparentsubstrate 1 and the transparent electrode 3 and the surface of the firsttransparent substrate 2 and the transparent electrode 4, respectively;and each of the alignment layers 5 and 6 is rubbed in its predetermineddirection. In the case of FIG. 1, rubbing is carried out in thedirection of the arrow a shown in the figure. In addition, rubbing mayalso be performed directly on the surface of a transparent substrate, onwhich no alignment layer is formed. Furthermore, concerning thedirection of rubbing, the rubbing direction is shifted between the firstalignment layer 5 and the second alignment layer 6 according to atwist-angle when a liquid crystal and a polymer are rendered in atwisted state, as is mentioned in the latter.

The solution mentioned below compatibly dissolving a liquid crystal, apolymer precursor, and a specific additive is sealed in a liquid crystalcell composed of the two transparent substrates 1 and 2 which are heldat a predetermined distance (e. g., 5 to 10 fEm) using a sealant orspacers not shown in the figure.

A liquid crystal having refractive anisotropy and dielectric anisotropy,for example, a nematic liquid crystal, is used for this solution. As thepolymer precursor, those which are compatibly dissolved and dispersedwith a liquid crystal and which result in a solution exhibiting a liquidcrystal phase are used. In addition, the polymer precursor is a monomerforming a polymer due to polymerization; and the preferred examplesthereof are those polymers, into which a benzene skeleton, preferably abiphenyl skeleton, a terphenyl skeleton, or a quarterphenyl skeleton isintroduced, and derivatives thereof. Moreover, polymers not havingbenzene skeletons can be similarly used when they can be aligned with aliquid crystal, in other words, as long as they have similar refractiveanisotropy to liquid crystals. When a polymer has a benzene skeleton,the refractive index of the polymer can be adjusted by changing, forexample, the number of phenyl groups or the type of substituents bondingto the phenyl group.

Practical examples of polymer precursors are esters of biphenylmethanolor naphthol and methacrylic acid or acrylic acid, and derivativesthereof. In addition, ester derivatives of biphenol and methacrylic acidor acrylic acid may be mixed therewith. Furthermore, a-methylstyrene,epoxy compounds, and the like may be used as other polymer precursors.

According to a method of phase-separating a liquid crystal and a polymerunder dispersed conditions, a liquid crystal and a polymer precursor arecompatibly dissolved once and then formed into a polymer by polymerizingthe polymer precursor. For this polymerization, photo-polymerization ata predetermined temperature is preferably employed, as is mentionedlater.

In this embodiment, a specific additive, that is, a compound in which atleast one hydrogen atom in a benzene ring is substituted by a hydroxygroup or a compound having a benzene ring with a hydroxy group as abasic skeleton, is added to the above-mentioned solution prepared bycompatibly dissolving a liquid crystal and a polymer precursor. Examplesof such compounds are divalent phenols, such as hydroquinone, catechol,and resorcinol; and monovalent, trivalent, or higher than trivalentphenols in which hydrogen atoms in a benzene ring are similarlysubstituted by a hydroxy group. Furthermore, compounds in which othersubstituents bond to these phenol compounds are included in theabove-mentioned compounds, for example, compounds having the followingchemical formula 1 or 2. An example of compounds having the chemicalformula 1 is ditert-butylhydroquinone (hereinafter referred to as "BHQ")and an example of compounds having the chemical formula 2 isparatertbutylcatechol. ##STR1## wherein m+n=3 and m'+n'=3. ##STR2##wherein m"+n"=3.

The solution to which the above-mentioned compound has been added issealed in a liquid crystal cell and then a polymer precursor within thecell is polymerized. In practical mass production processes, it isfrequently impossible to carry out photo-polymerization immediatelyafter sealing the solution in a liquid crystal cell. In such cases, thepresent invention is advantageous. For example, an electro-opticaldevice without any deteriorated characteristic can be manufactured, evenwhen a mixture of a liquid crystal, a polymer precursor, and an additiveis photo-polymerized two weeks after leaving the mixture sealed in aliquid crystal cell. When polymerization is carried out approximately 1week after sealing a liquid crystal/polymer precursor without employingany additive such as BHQ used in the present invention, scatteringperformance of the resulting electro-optical device decreases.

Although heat polymerization, photo-polymerization, or the like can beemployed as the polymerization method, in general, photo-polymerizationis preferable considering liquid crystal materials. The most frequentlyused method is such that ultraviolet radiation is employed forpolymerization using an ultraviolet curing polymer. According to thismethod, since the intensity of light, radiation time, and environmentaltemperature of ultraviolet radiation affect the driving and opticalcharacteristics of liquid crystal displays, the intensity of light,radiation time, and environmental temperature required for obtainingpreferable characteristics are predetermined by experiment. The lowerlimit of the radiation time is set to a time during which the amount ofmonomer left in the resulting liquid crystal polymer complex layer issufficiently reduced. In addition, the polymer precursor content withrespect to the solution is preferably 3 to 10% by weight, and morepreferably, 5 to 8% by weight. Polymerization initiators can be used forpolymerization, depending on necessity.

A polymer produced by photo-polymerization in this polymerizing processphase-separates from the solution, thus when polymerization proceeds andfinally leads to a decrease in the amount of residual monomer, a liquidcrystal polymer complex layer 7 is formed in which numerous polymerparticles 7B are dispersed in a liquid crystal 7A, as is shown in FIG.1.

Since the liquid crystal polymer complex layer 7 produced as above is incontact with alignment layers 5 and 6 rubbed in the direction of arrow ain the figure, it is fundamentally aligned in the direction of arrow a.This is because a solution exhibiting a liquid crystal phase before apolymerization process is in contact with the alignment layers 5 and 6.Thus, molecules of the liquid crystal and the polymer precursor in thesolution are aligned beforehand along the rubbing direction of thealignment layers 5 and 6, and a polymer is formed byphoto-polymerization while maintaining the alignment direction after theabove-mentioned polymerization process. For rendering a liquid crystaland a polymer in a twisted state, the upper portion of the liquidcrystal polymer complex layer 7 is aligned in the rubbing direction ofthe first alignment layer 5 and the lower portion of the liquid crystalpolymer complex layer 7 is aligned in the rubbing direction of thesecond alignment layer 6 so that a predetermined twist angle is formed.

In this condition, that is, a condition in which the liquid crystal 7Aand the polymer particles 7B are aligned in the same direction withoutapplying a voltage to the transparent electrode 3 and the metallicelectrode 4, as is above-mentioned, the liquid crystal 7A and thepolymer particles 7B are set to have almost the same refractive index.Therefore, the liquid crystal polymer complex layer 7 is in atransparent state when no electric field is applied thereto.

However, when a voltage not less than a predetermined threshold voltageis applied between the transparent electrode 3 and the metallicelectrode 4, only the liquid crystal 7A having dielectric anisotropy isrendered in a different alignment state by the electric field, as isshown in FIG. 2. In this case, if the liquid crystal has positivedielectric anisotropy, the liquid crystal 7A is aligned in the directionof the electric field, as is shown in the figure. Since the liquidcrystal 7A has refractive dielectric anisotropy, the refractive indexesof the liquid crystal 7A and the polymer particles 7B differ from eachother when the alignment direction of the liquid crystal 7A changes.Thus, the liquid crystal polymer complex layer 7 is rendered in alight-scattering state in which light entering through the firsttransparent substrate 1 is scattered.

Incidently, this type of liquid crystal display generally deterioratesin transparency when exposed to certain high temperatures or afterimageoccurs so that the display remains in an ON state when left in the ONstate for a long time period. Therefore, the driving and opticalcharacteristics after heat-aging treatment (heat-aging characteristics),in which a liquid crystal display is held for a long time period whileheated to a predetermined temperature, and the driving and opticalcharacteristics after energize-aging treatment (energize-agingcharacteristics), in which a liquid crystal display is held for a longtime period while a predetermined driving voltage is applied thereto,are important.

In practical use, both the above-mentioned heat-aging treatment andenergize-aging treatment are supposed to have influence, thus it isnecessary for evaluation to employ both of them in combination.According to a liquid crystal display of the present invention, theeffect due to heat-aging treatment is small, and further, theenergize-aging characteristics are improved compared with conventionalliquid crystal displays, i. e., those to which one of theabove-mentioned additives is not added.

Although the amount of the above-mentioned additive varies according tothe type of additive, 0.0005 to 1.0% by weight with respect to thesolution is preferable. When the amount of additive is less than thisrange, an effect by its addition cannot be obtained. Meanwhile, if theadded amount exceeds the above range, polymerization of the polymerprecursor is suppressed, thereby a long time period is occasionallyrequired for the polymerization steps or deterioration in the driving oroptical characteristics occurs. This is because the above-mentionedadditives function as polymerization suppressors for polymerizingpolymers incorporated in the present invention.

The added amount of the above-mentioned additive is is also limited byits solubility in the above-mentioned solution. For example, as comparedwith the above-mentioned hydroquinone, the above-mentioned BHQ has ahigher solubility in a typical used solution and the added amountthereof is more easily adjusted.

Moreover, the type of the above-mentioned additive causes variouslimitations. For example, even with a trace amount added, as ismentioned above, certain types of compounds may affect the displaycharacteristics. Thus, it is necessary to change the compound or limitthe added amount in such cases. Examples of compounds having smallinfluence on the display characteristics are the above-mentionedhydroquinone and BHQ.

Next, more practical examples of the present invention will beexplained.

EXAMPLES Example 1

Each of the solutions used as the above-mentioned solution was preparedsuch that 7% by weight of a polymer precursor was compatibly dissolvedin a liquid crystal and BHQ was further added thereto at 0% by weight,0.001% by weight, 0.01% by weight, 0.1% by weight, and 1% by weight, andevaluation was carried out on the resulting solutions. TL215B (ProductNo., manufactured by Merck Japan Limited) was used as a liquid crystaland a mixture of biphenylmethacrylate and bisphenol A dimethacrylate ata weight ratio of 11:1 was used as a polymer precursor. Although thefollowing results were obtained from experiments using liquid crystalcells composed of these materials, similar results were obtained whenBL007 (Product No., manufactured by Merck Japan Limited) was used as aliquid crystal and 6% by weight of a mixture of terphenylmethacrylateand p,p"-biphenyldimethacrylate at a weight ratio of 4:1 was addedthereto as a polymer precursor.

A 5 mm thick cell in which a chromium electrode was formed as themetallic electrode 4 and the twist angle of the liquid crystal polymercomplex layer 7 was set to 90° was used as the liquid crystal cell. Theabove-mentioned solution was sealed in this liquid crystal cell andphoto-polymerized by ultraviolet radiation. In the photo-polymerizingprocess, liquid crystal polymer complex layers 7 were formed at anenvironmental temperature of 50° C. under either of the following twoconditions: radiation at a light intensity of 3 mW for 10 min. (case 1)and radiation at a light intensity of 20 mW for 200 seconds (case 2).

The thus produced samples of case 1 and case 2 were first subjected toheat-aging treatment at 70° C. The heating time was 15 hours for thesample of case 1 and 120 hours for the sample of case 2. The sampleswere then subjected to energize-aging treatment at 50° C. for 15 hours.The applied voltage was 5 V for the sample of case 1 and 4 V for thesample of case 2, considering the relation to the threshold voltage.

FIG. 3 shows a dependency of the refractive index on the added amount ofBHQ when applying a voltage (an ON state) in the above-mentioned cases 1and 2 after heat-aging treatment and energize-aging treatment. In FIG.3, case 1 is indicated by a and case 2 is indicated by b. Furthermore,FIG. 4 shows a dependency of contrast on the added amount of BHQ. InFIG. 4, case 1 is indicated by a and case 2 is indicated by b.

It is understood from FIGS. 3 and 4 that the refractive index andcontrast are improved when certain amounts of BHQ are added as comparedwith those without addition. Although varying according to conditions,an effective added amount of BHQ ranges from 0.0005 to 0.1% by weightwhen subjected to the above-mentioned aging, more preferably, from 0.001to 0.1% by weight, and further more preferably, from 0.001 to 0.01% byweight.

FIG. 5 shows a dependency of the amount of residual monomer in theliquid crystal polymer complex layer 7 of case 1 on the polymerizingtime for each added amount of BHQ. When the amount of residual monomeris large, both the driving characteristics and the opticalcharacteristics deteriorate, thereby reducing stability of the liquidcrystal polymer complex layer. It is understood from this figure thatuntil the added amount of BHQ reaches 0.1% by weight, the polymerizingspeed and the final amount of residual monomer do not greatly change,and when the added amount of BHQ exceeds 0.1% by weight, thepolymerizing speed decreases and the amount of residual monomerincreases. Therefore, the upper limit of the added amount of BHQ isapproximately 0.1% by weight from the viewpoint of polymerizing speedand amount of residual monomer.

Although afterimage and a reduction in contrast associated with theenergize-aging can be suppressed if a liquid crystal polymer complexlayer is formed by adding BHQ to a solution, as is above mentioned,clear reasons for this phenomenon have not yet been obtained. However,the above-mentioned additives function as polymerization suppressorswhen a certain amount thereof is added. Thus, it is possible that theyalter the polymerization conditions of polymer precursors. Electronmicroscopic photographs of the liquid crystal polymer complex layers ofthe above-mentioned example show the following: in samples exhibitingclear effects, to which samples 0.001% by weight or 0.01% by weight ofBHQ was added, a small gel portion in the shape of a film covering thepolymer is observed and clear polymer particles are visually confirmed;meanwhile, in samples prepared without adding BHQ, a large gel portionis observed and the linked polymer particles are covered with afilm-like material. The polymer particles are in the shape of particleshaving an almost oval sectional form. Although the size of the polymerparticles varies according to monomers and reaction conditions, forexample, the size is 0.1 to 0.5 mm in the uniaxial direction and 2 to 10mm in the direction of major axis.

Example 2

In this Example, a mixture of BL007 manufactured by Merck Japan Limitedand RDP50614 manufactured by RODIC Co., Ltd. at a weight ratio of 1:1was used as a liquid crystal, and 0.3% by weight of R1011 manufacturedby Merck Japan Limited was added thereto as a chiral component. Withrespect to the liquid crystal, 6% by weight of a mixture offluoroterphenylmethacrylate and biphenyldimethacrylate at a weight ratioof 4:1 was used as a polymer precursor. The relationship between theadded amount of BHQ and the electro-optical characteristics wasinvestigated in this material system. A 5 mm thick cell was used whichhad an alignment layer thickness of 30 nm and an alignment direction of180°. The above-mentioned mixture of liquid crystal/polymer precursorand BHQ were sealed in the empty cell and polymerized by black lighthaving an ultraviolet intensity of 3.3 mW/cm² (350 nm) at 50° C. for 10min. so as to manufacture an electro-optical device. The BHQconcentration varied between 0 and 0.1% by weight.

FIG. 6 shows the relationship between the BHQ concentration and thedriving voltage and FIG. 7 shows the relationship between the BHQconcentration and the refractive index in the ON state. In FIG. 6, aline indicated by a shows changes in the threshold voltage and a lineindicated by b shows changes in the saturated voltage. In FIG. 7, a lineindicated by a shows changes in the refractive index at an ON voltage of4.4 V and a line indicated by b shows changes in the refractive index ata saturated voltage.

It is understood from FIG. 6 that both the threshold voltage and thesaturated voltage are almost constant when the BHQ concentration is lowand the threshold voltage and the saturated voltage gradually increasewhen the BHQ concentration is greater than 0.01% by weight. In addition,it is understood from FIG. 7 that the refractive index at the ON andsaturated voltages gradually increases with a rise in the BHQconcentration and reaches a maximum value when the BHQ concentration is0.05% by weight.

Example 3

This example was carried out similarly to Example 2, except thathydroquinone (HQ) was employed instead of BHQ used in Example 2. Therelationship between the added amount of HQ and the electro-opticalcharacteristics was investigated in a similar manner to Example 2.

FIG. 8 shows the relationship between the HQ concentration and thedriving voltage and FIG. 9 shows the relationship between the HQconcentration and the refractive index in an ON state. In FIG. 8, a lineindicated by a shows changes in the threshold voltage and a lineindicated by b shows changes in the saturated voltage. In FIG. 9, a lineindicated by a shows changes in the refractive index at an ON voltage of4.4 V and a line indicated by b shows changes in the refractive index ata saturated voltage.

It is understood from FIG. 8 that both the threshold voltage and thesaturated voltage are almost constant even when the HQ concentrationchanges. In addition, it is understood from FIG. 9 that the refractiveindexes at the ON voltage and the saturated voltage increase in a BHQconcentration range of 0.005 to 0.1% by weight.

What is claimed is:
 1. A polymer dispersed liquid crystalelectro-optical device, comprising:a liquid crystal polymer complexlayer comprising a liquid crystal material and a polymer, wherein saidliquid crystal material and said polymer are aligned in the samedirection when no electric field is applied; and an electrode structureformed on each side of said liquid crystal polymer complex layer forapplying an electric field to said liquid crystal polymer complex layerto align said liquid crystal material along the electric field so as torender said liquid crystal polymer complex layer in a light-scatteringstate; wherein said liquid crystal polymer complex layer is formedby:dissolving a liquid crystal material and a polymer precursor to forma solution; adding butylhydroquinone to said solution in a range of0.0005-0.1% by weight with respect to said solution;polymerizing saidpolymer precursor to form a polymer; and phase separating said liquidcrystal material and said polymer to form a liquid crystal polymercomplex layer.
 2. A method for manufacturing a polymer dispersed liquidcrystal electro-optical device, comprising:dissolving a liquid crystalmaterial and a polymer precursor to form a solution; addingbutylhydroquinone to said solution in a range of 0.0005-0.1% by weightwith respect to said solution; radiating said solution with light,thereby photo-polymerizing said polymer precursor to form a polymer; andphase separating said liquid crystal material and said polymer to form aliquid crystal polymer complex layer.
 3. A polymer dispersed liquidcrystal electro-optical device, comprising:a liquid crystal polymercomplex layer comprising a liquid crystal material and a polymer,wherein said liquid crystal material and said polymer are aligned in thesame direction when no electric field is applied; and an electrodestructure formed on each side of said liquid crystal polymer complexlayer for applying an electric field to said liquid crystal polymercomplex layer to align said liquid crystal material along the electricfield so as to render said liquid crystal polymer complex layer in alight-scattering state; wherein said liquid crystal polymer complexlayer is formed by:dissolving a liquid crystal material and a polymerprecursor to form a solution; adding hydroquinone to said solution in arange of 0.005-0.1% by weight with respect to said solution;polymerizing said polymer precursor to form a polymer; and phaseseparating said liquid crystal material and said polymer to form aliquid crystal polymer complex layer.
 4. A method for manufacturing apolymer dispersed liquid crystal electro-optical device,comprising:dissolving a liquid crystal material and a polymer precursorto form a solution; adding hydroquinone to said solution in a range of0.005-0.1% by weight with respect to said solution; radiating saidsolution with light, thereby photo-polymerizing said polymer precursorto form a polymer; and phase separating said liquid crystal material andsaid polymer to form a liquid crystal polymer complex layer.